Image placement help

I've never used codes other than the really really standard stuff.

I'm using the stylesheet "The Earth's Story Illustrated" from This website. The problem is I can't figure out how to add a photo into the box at the top.

It says this is how to add one:

This stylesheet is capable of displaying a 480-pixels-tall scene image above every passage's text! You can set scene images using Tag CSS. To assign, say, the image "classroom_afternoon" to the tag "classroom", simply create a stylesheet tagged "stylesheet classroom" and put this in it:
.passage .header {
background-image: [img[classroom]];
}
And then tag various passages with "classroom" to use the image.
To have multiple images overlaid on each other, add multiple img declarations separated by commas, in order from frontmost to rearmost:
.passage .header {
background-image: [img[character-funnycry]], [img[classroom]];
}


But I have no idea what any of this means. I watched a tutorial on how to add an image using html, but when I do it it places the image into the box where the text goes. I'm using twine online if that's worth mentioning.
Thankyou in advance!